Have you checked Settings > General > Storage & iCloud Usage > Storage for Facebook? If you can find it, you may be able to delete it from there.
To deactivate and delete it, follow these steps: 1. Open the Facebook app on your device. 2. Tap the three lines to the right of the top navigation bar. 3. Continue to Account Deletion by tapping Delete Account from the drop-down menu.
Select Settings & Privacy from the drop-down menu at the top right of Facebook. Then, under Settings & Privacy, click Settings. In the left column, select Your Facebook Information. Choose Deactivation and Deletion from the drop-down menu. Select Permanently Delete Account from the list and then Continue to Account Deletion.
Select “Accounts” from the Settings menu. Select “Accounts & Apple ID” from the drop-down list. Enter your device’s passcode at the prompt to confirm that you wish to deactivate and delete it. Select Delete Account, then Continue to Account Deletion from this screen. Tap Deactivation and Deletion in the Management window for more information regarding device management options for mobile devices. For instructions on how to remove an account using a Mac computer, go here .
Because Facebook is a system app on your phone, the Facebook app is preinstalled. You won’t be able to uninstall it, but you will be able to disable it. Go to Settings > Applications manager and look for Facebook.